This application is based on and claims priority from Japanese Patent Application Hei. 2-93802, filed Apr. 9, 1990, the disclosure of which is incorporated by reference herein.
This invention relates to a light-admitting device and, more particularly, relates to a light-admitting device suitable for use in a photodetecting apparatus such as a white balance sensor.
Automatic white balance sensors used in electronic still cameras and other imaging apparatus are adapted to operate in the following way. The ratios of respective light components, i.e., R (red), G (green) and B (blue) components, obtained through an RGB filter assembly provided on the light-receiving surface are detected and the resulting detection signals (e.g. R/G and B/G) are used to control the gains of amplifier circuits for color signals contained in the video signal output from the imaging device in such a way that the image of a white object will be reproduced white under any kind of light source, thereby achieving optimal color reproduction in accordance with the color temperature of a light source.
FIG. 3 is a typical circuit diagram for electronic still cameras and other imaging apparatus that are adapted to achieve color reproduction associated with various color temperatures. The circuit shown in FIG. 3 includes an imaging device 20 typically formed of a CCD, a color separator circuit 21 for extracting red (R) and blue (B) primary color signal from each video signal from the imaging device 20, and a luminance signal processing circuit 22 for separating a luminance signal Y.sub.L from each video signal from the imaging device 20. The R and B signals issuing from the color separator circuit 21 are amplified by associated amplifier circuits 23 and 24 and are respectively supplied into a .gamma.-correction/matrix circuit 25. The luminance signal Y.sub.L issuing from the luminance signal processing circuit 22 is also supplied into the .gamma.-correction/matrix circuit 25. The circuit 25 outputs two color-difference signals, R-Y and B-Y, as well as the luminance signal Y.sub.L, all of which serve to generate an NTSC color television signal.
Shown by 26 in FIG. 3 is an automatic white balance sensor which outputs R, G and B signals associated with respective color temperatures. Those signals are converted to logarithmic values by means of a logarithmic converter circuit 27. Of the three logarithmic signals, those for log R and log G are supplied to a first differential voltage generating circuit 28 which constructs an R white balance control voltage R/G from the differential voltage for the two input signals. The logarithmic signal for log B from the converter circuit 27 is supplied together with the signal for log G to a second differential voltage generating circuit 29 which constructs a B white balance control voltage B/G from the differential voltage for the two input signals.
The two control voltages R/G and B/G thus produced from the differential voltage generating circuits 28 and 29 are fed into the associated R and B amplifier circuits 23 and 24, and the gains of the respective amplifiers are controlled in such a way that the image of a white object will be produced white under any kind of light source.
The white balance sensor used in the system described above must be so designed that the RGB light-receiving region is illuminated uniformly irrespective of the angle of incidence of light falling on that region. In the prior art, the combination of a light-diffusing plate filter and a luminosity compensating filter has been provided on the light-receiving surface of the sensor so that uniform illumination will occur in the RGB light-receiving region. However, in the prior art automatic white balance sensor having the construction described above, the light-receiving surface of the light-diffusing plate is parallel and in proximity to that of the sensor, and any change in the angle of incidence of light falling on the diffusing plate or any directivity of the incident light will directly affect the light-receiving surface of the sensor so that it will not be illuminated uniformly with the incident light.
Particularly, in the case where the RGB color filter assembly provided on the light-receiving surface of the sensor is a striped type, uneven illumination occurs on the light-receiving surface if incident light is admitted sideways, i.e., in a direction normal to the stripes. One may consider increasing the thickness of the light-diffusing plate in order to achieve even illumination, but then less light is admitted by the light-receiving surface of the sensor and the level of the output from the automatic white balance sensor will decrease to make it highly susceptible to external noise.